Different fillers have been generally used. Among others, calcium carbonate is advantageous for the following reasons that optical characteristics can be imparted to paper, and its price is lower than that of paper stock. Therefore, movement into neutral papermaking is presently advanced, and hence the amount of calcium carbonate in paper tends to gradually increase. However, the increased amount of calcium carbonate in the paper causes deterioration in paper strength and sizing performance. Consequently, a wider use of calcium carbonate can be expected by solving the above problems, namely, paper strength deterioration and sizing performance deterioration.
In the recent papermaking method using a filler, particularly a large amount of calcium carbonate, when a reactive sizing agent such as AKD (alkyl ketene dimer) and ASA (alkenyl succinic anhydrides) is used as an internal sizing agent, the following problem remains. That is, a larger amount of the sizing agent causes more dirt of papermaking system, whereas a smaller amount of the sizing agent for preventing the dirt fails to obtain high sizing performance. When a dispersed rosin sizing agent is used as an internal sizing agent, it is necessary to enhance sizing performance by increasing the amount of addition of aluminum sulfate. In this case, due to lowing of pH of the papermaking system, calcium carbonate dissolves, and gypsum is deposited as calcium scale at various parts of a paper machine, thus susceptible to occurrence of dirt.
For the purpose of improving the optical characteristics of the paper, the retention of the filler itself or the paper strength, for example, the following techniques for pretreating calcium carbonate with various kinds of polymers have been proposed.
(1) Patent Document 1
It is described that the filler retention deterioration and the paper strength deterioration are reduced by adding a cationic calcium carbonate after being subjected to coating adsorption treatment with a cationic polymer or an amphoteric polymer, to pulp slurry (refer to claim 1, and paragraph “0007”). As example 1, it is described that precipitated calcium carbonate is subjected to coating treatment with a water-soluble polymer composed of dimethylaminoethyl acrylate and acrylamide, and then added to pulp slurry (refer to paragraph “0017”).
(2) Patent Document 2
It is described that a filler is treated with starch and a high molecular weight organic material (for example, polyacrylamide (PAM)) (refer to claims).
(3) Patent Document 3
It is described that sizing performance deterioration can be reduced even at a high filler content by using a filler product obtained by bringing a liquid, in which a cellulose reactive sizing agent such as AKD and ASA is dispersed in water with a dispersing agent such as cationic starch, into contact with a filler such as calcium carbonate (refer to claim 1, and paragraphs “0007” and “0011”).
(4) Patent Document 4
It is described that the required amount of sizing agent can be decreased by using the filler treated with cationic modified AKD (preferably PCC (precipitated calcium carbonate)) (refer to paragraph “0005”).
(5) Patent Document 5
It is described that the adsorption of an internal sizing agent can be reduced by using the filler (calcium carbonate, China clay, titanium oxide, etc., refer to claim 2) which is coated with C12 to C22 water-soluble fatty acid salts (preferably, sodium stearate, refer to claim 6 and page 8) in coexistence of metal ions (the ions of aluminum, barium, lithium, magnesium, etc., refer to claim 7 and pages 7 to 9) (refer to claims 1 to 6).
Patent document 1: Japanese Unexamined Patent Publication No. 4-281094
Patent document 2: Japanese Unexamined Patent Publication No. 56-49097
Patent document 3: Japanese Unexamined Patent Publication No. 4-228697
Patent document 4: Japanese Unexamined Patent Publication No. 5-247886
Patent document 5: Japanese Unexamined Patent Publication No. 8-507837